1. Field of the Invention
The present invention relates to a technique for distinguishing between recordable optical discs and read-only optical discs.
2. Description of Related Art
In recent years, in an optical disc field, recordable optical discs with reproduction compatibility with read-only optical discs have been developed. Since recordable optical discs have recording capacity equivalent to read-only optical discs, they have been attracting people's attention and gaining popularity as recording media that satisfy growing requests for increased capacity in recent years. In addition, although there exist recordable optical discs which have no reproduction compatibility with read-only optical discs, many of recording/reproduction apparatus are configured to reproduce both optical discs. For examples of these types of recordable optical discs, DVD-R/RW and DVD-RAM are known.
On the other hand, as the popularity of recordable optical discs increases, a problem has occurred that information recorded in read-only optical discs is copied into recordable optical discs as it is and thus, a copyright related to the information is infringed. For measures against this problem, a technique to add ID information to read-only optical discs to distinguish between read-only optical discs and recordable optical discs by determining whether discs have the ID information or not has been thought up. However, according to the technique, it is impossible to cope with the case in which ID information is copied.
Therefore, another technique for distinguishing between read-only optical discs and recordable optical discs has been developed, in which discs are determined, not by ID information, but by a difference of physical characteristics of optical discs themselves. To specifically explain physical characteristics of optical discs, first of all, in read-only optical discs, pits and spaces are arranged along spiral data recording tracks. By the pits and spaces, information is recorded. On the other hand, in the recordable optical discs, spirally built grooves or lands intervened between grooves, recording marks and spaces are arranged. Information is recorded by the recording marks and spaces. The grooves of recordable optical discs are formed in wobbling form at a frequency one-several-hundredth of data reproduction speed, and recording clocks are generated based on the frequency. In addition, by the groove wobbling, address information on the optical disc is embedded.
The difference between read-only optical discs and recordable optical discs lies in whether the wobbling tracks as described above exist on the optical disc or not. Therefore, by detecting such wobbling tracks, it is possible to judge whether the disc is recordable or read-only. If an optical disc to be reproduced is judged to be recordable one, and on the recordable optical disc, information whose copyright must be protected is recorded, reproduction of the information on the disc is prohibited. Therefore, copyright can be definitely protected.
Furthermore, because optical disc drives of recent years can record/reproduce information for optical discs of various format specifications, it is extremely useful to distinguish between read-only optical discs and recordable optical discs from viewpoints other than copyright protection. For this distinguishing technique, a technique for detecting wobbles of grooves located on recordable optical discs (see Japanese Laid-open Patent Publication No. 4-123320) and a technique for determining recordable optical discs by measuring the groove wobbling frequency (see Japanese Laid-open Patent Publication No. 2000-322742) have been known.
FIG. 9 is a block diagram showing a configuration of conventional reproduction apparatus 50. Reproduction apparatus 50 detects whether data recording track 2 on optical disc 1 wobbles or not. Reproduction apparatus 50 includes disc motor 3 which rotates optical disc 1, optical head 4 which irradiates optical disc 1 with optical beam and receives reflected light, light receiving element 5 which is located inside optical head 4 and receives reflected light from optical disc 1, transfer mechanism 6 which transfers optical head 4 in radial direction of optical disc 1, and linear speed control section 7 which controls reproduction linear velocity by controlling disc motor 3 and transfer mechanism 6. Reproduction apparatus 50 further includes push-pull signal generator 8 which generates a push-pull signal based on a difference of output signals of light-receiving element 5, band pass filter 9 that only passes a specified frequency component of the push-pull signal outputted from push-pull signal generator 8, clock generator 11 which supplies measured clocks to frequency measuring counter 10, and arithmetic section 12 which carries out arithmetic processing on periodic data measured by period measuring counter 10.
In the push-pull signal generated by push-pull signal generator 8 of reproduction apparatus 50, wobbling frequency component is contained in the case that data recording track 2 wobbles. Band pass filter 9 extracts this frequency component. Reproduction apparatus 50 further includes judging section 13 which determines whether data recording track 2 wobbles or not by the arithmetic results of arithmetic section 12 and outputs the judgment results. Based on the judgment results, the judging section determines whether data recording track 2 on optical disc 1 wobbles or not, that is, whether optical disc 1 is recordable or read-only.
However, in the event that the recordable optical disc is, for example, DVD-R/RW, there are some cases in which reproduction apparatus 50 erroneously judges the recordable optical disc to be read-only or in which reproduction apparatus 50 is unable to judge a type of the disc. The reasons are as follows. First of all, because in DVD-R/RW, pits called land pre-pits which are formed at lands exist, the push-pull signal output cannot be detected as a specified sine wave. In addition, when recording marks are formed in grooves, a signal-to-noise ratio (S/N ratio) of the push-pull signal is extremely degraded due to influences of the recording marks. Consequently, jitters of output from band pass filter 9 increase. Then, a difference between jitters of the recordable optical discs and read-only optical discs decrease. Consequently, it becomes difficult to set a threshold value for distinguishing between the recordable optical discs and the read-only optical discs.
Furthermore, in the event that there is large eccentricity in an optical disc, reproduction linear velocity changes due to the eccentricity. Then, the results of period measurement greatly vary according to phase of eccentricity, and setting of judgment threshold value becomes still more difficult or in some cases, becomes impossible.